Multiple antenna processing on transmit for wireless local area networks

ABSTRACT

A method and an apparatus in a first wireless station of a network transmitting to a second wireless station. The network uses multi-tone OFDM signals. The first station includes multiple antennas and a receive and a transmit signal path per antenna. Each receive signal path includes a discrete Fourier transformer determining the tones in a received signal, and each transmit signal path includes an inverse discrete Fourier transformer converting tones to a signal. The method includes determining channel estimates for each tone and each receive path while receiving from the second station, determining transmit weights to transmit to the second station, tone-by-tone weighting a signal for transmission to the second station to produce weighted tone sets for each transmit signal path, and transmitting the weighted tone sets. The first station is configured so that the weighting produces additive beamforming without the second station needing multiple antennas.

BACKGROUND

The present invention is related to wireless networks, and in particularto methods and apparatuses for transmitting using multiple antennaswithout requiring the receiver of the transmission to have multipleantennas.

Wireless networks, e.g., local area wireless networks (WLANs) conformingto the IEEE 802.11 standard have become common. It is known that theperformance of a link in such a WLAN is significantly degraded in thepresence of multipath, as in an office setting where there is noline-of-sight from the client to the access point. Some variants of theIEEE 802.11 standard use orthogonal frequency division multiplexing(OFDM), which is known to perform better than many alternatives in thepresence of multipath.

WLANs often are used in an infrastructure wherein one wireless stationof the network, called an access point, acts as a base station for a setof client stations. One mechanism for improving communication is to usemultiple antennas at the access point and possibly at the clientstations.

It is known, for example, to use antenna selection diversity at theaccess point wherein one of a plurality of receive antennas is selectedaccording to a selection criterion, typically signal strength at the tworeceivers as measured by the received signal strength indication (RSSI)signal at the radio receiver. U.S. patent application Ser. No.10/698,588 to Lyons et al. filed Oct. 31, 2003 and titled ERROR VECTORMAGNITUDE SELECTION DIVERSITY METRIC FOR OFDM, Attorney/Agent Docket No.CISCO-7727, introduced an alternate measure for antenna selection in anOFDM receiver based on an error vector magnitude (EVM) measure obtainedat the receiver and measured from a preamble part of a packet as used inWLANs and received at the receiver.

It also is known to use beamforming at the access point, e.g., to usemultiple radio receivers, one per receive antenna, and then combine thereceived signals from each antenna according to a combining method.

These methods significantly improve reception at the access point. Ofcourse one can similarly improve reception at the client fortransmissions by the access point by including multiple antennas at theclient station. It would be beneficial, however, for the client toremain single antenna to maintain lower cost.

One known method of maintaining single antenna clients while havingsymmetry in the quality of reception at the access point (the uplinkdirection) and the quality of reception at the client (the downlinkdirection) is to include receive diversity at the access point foruplink improvement and transmit at higher power on the downlink. Thehigher transmit power, however, increases the likelihood of co-channelinterference in an environment that includes several access points.

Thus there is a need in the art for methods of transmitting usingmultiple transmit antennas.

One known multiple antenna transmit solution includes changing whichtransmit antenna is used when a packet fails to be received at theclient. This technique effectively involves transmit selection diversityat the media access control (MAC) level.

Thus there is still a need in the art for methods and apparatuses oftransmitting using multiple transmit antennas.

There further is a need in the art for methods and apparatuses oftransmitting using multiple transmit antennas that do not require thereceiver, e.g., a single antenna client, to exchange knowledge relatedto calibration.

SUMMARY

Described herein is an apparatus to operate in a first wireless station,e.g., an AP of a wireless network, to transmit to a second wirelessstation, e.g., a client station of the AP. Also described herein is amethod in the first wireless station for transmitting to the secondstation. The first and second stations are for communicating packets ofinformation using OFDM signals that include a plurality of frequencytones, e.g., conforming to one of the OFDM variants of the IEEE 802.11standard. The first station including a plurality of antennas, e.g., twoantennas for receiving and transmitting coupled to a correspondingplurality of receive signal paths for receiving and to a correspondingplurality of transmit signal paths for transmitting.

One embodiment of the method includes determining the channel responsefor each receive signal path. Such channel response determining usessignals received at the first station corresponding to a part of apacket transmitted from the second station. That part of the packet hasknown values for a set of tones. The channel response determiningincludes performing a discrete Fourier transform to determine receivedtones corresponding to the part of the packet, and generating channelestimates for the receive signal paths for each tone whose value isknown in the part of the packet.

The method also includes determining a set of transmit weights for eachtone for each transmit signal path using the channel estimates, andtone-by-tone weighting a signal for transmission by the set ofdetermined transmit weights to produce weighted tone sets fortransmission via each transmit signal path.

The method also includes inverse discrete Fourier transforming theweighted tone sets to produce antenna signals for transmission via thetransmit signal paths, and transmitting the set of antenna signals fortransmission via each transmit signal path via the antennas.

The first station is configured such that the channel response at thereceiving second station includes an additive contribution fortransmissions via each transmitting antenna of the first station. Themethod is such that the second station can receive the signal fortransmission without the second station requiring a plurality of receiveantennas and without any first-station-specific calibration required atthe second station.

In one embodiment, the transmit weight for each transmit signal pathcorresponding to each antenna has a phase angle which is the negative ofthe phase angle of the determined channel response for correspondingreceive signal path connected to the same antenna.

In one implementation, each transmit signal path of the first stationincludes a transmit digital signal path whose output is coupled to adigital-to-analog converter whose output is coupled to a transmit RFsignal path coupled to the antenna corresponding to the transmit signalpath. Furthermore, each receive signal path of the first stationincludes a receive RF signal path coupled to the antenna correspondingto the receive signal path, and the output of each receive RF signalpath is coupled to an analog-to-digital converter whose output iscoupled to a receive digital signal path. The magnitude and phaseresponse of the transmit digital signal path is substantially the samefor each transmit signal path, and the magnitude and phase response ofthe receive digital signal path is substantially the same for eachreceive signal path. One aspect of the invention is configuring thefirst station such that the magnitude and phase response of each RFtransmit signal path is substantially equal. Another aspect of theinvention configuring the first station such that phase response of eachRF transmit signal path is substantially equal.

In an alternate version, the transmit weights determining and thetone-by-tone weighting together include, for each tone, selecting one ofthe transmit signal paths for transmitting the signal for transmitting.The selecting is according to the determined channel response that hasthe largest magnitude, such that for each tone, the selecting isequivalent to weighting the signal for transmitting via the selectedtransmit signal path by one, and weighting the signal for transmittingvia each other transmit signal path by zero.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a simplified WLAN that includes a client and anaccess point (AP) that implements an embodiment of the presentinvention. FIG. 1A shows the client transmitting and the access pointreceiving (the uplink), while FIG. 1B shows the access pointtransmitting and the client receiving (the downlink).

FIGS. 2A and 2B show a simplified WLAN that includes a client and anaccess point that implements an alternate embodiment using selectiondiversity, so it does not require the same amount of electronics at theAP as in the AP shown in FIGS. 1A and 1B.

FIGS. 3A and 3B show a simplified WLAN that includes a client and anaccess point that implements another alternate embodiment usingselection diversity on the downlink.

FIG. 4 is a simplified block diagram of a complete access pointembodiment shown in more detail than the block diagrams of FIGS. 1A and1B.

FIG. 5 is a simplified block diagram illustrating in more detail thanshown in FIG. 4 one embodiment of the coding and modulation unit, and ofthe post transmit beamformer processing.

FIG. 6 shows some mathematical terms contributing to the overall channelresponse for a tone for one transmit beamforming embodiment.

DETAILED DESCRIPTION

The present invention is described herein in the context of a WLAN thatconforms to one of the OFDM variants of the IEEE 802.11 standard. FIGS.1A and 1B show a simplified WLAN that includes a client 103 and anaccess point (AP) 105. FIG. 1A shows the client 103 transmitting and theaccess point 105 receiving (the uplink), while FIG. 1B shows the accesspoint 105 transmitting and the client 103 receiving.

The client is shown in simple form as having a digital modem part 107that for transmission accepts information from a MAC controller (notshown), and carries out digital modulation tasks according to thestandard, including scrambling to form scrambled information, encodingto encode the information, puncturing, interleaving to form interleavedcoded information, and modulating to form modulated OFDM tones, alsocalled subcarriers. The modulated OFDM tones are subject to an inversediscrete Fourier transform (IDFT) operation and are cyclically extendedto form the ODFM modulated digital signal to which a preamble is addedto form the OFDM digital samples. The digital samples are converted toanalog information via a digital-to-analog converter (DAC) to form theOFDM signal for transmission. The analog information is input to thetransmitter RF part of an RF transceiver 109 coupled to an antenna 111.The RF signal is thus transmitted to the AP 105.

The multiple antenna aspects of the present invention are describedherein using two antennas, and those in the art will understand thataspects of the invention may be extended to more than two antennas.

The access point 105 of FIGS. 1A and 1B includes two antennas 113 and115. The description herein assumes the same antennas are used at the APfor transmit and receive. Referring still to FIG. 1A, the transmittedsignal is received at each of the antennas 113 and 115 that are coupledto respective receiver parts of radio transceivers 117 and 119,respectively, via a duplexer, which in this time domain duplexing case,is a switch that connects to the receive or transmit parts depending onwhether the AP is receiving or transmitting. The analog outputs of thetransceivers are baseband, or close to baseband signals. In oneembodiment, each receive chain of the transceivers 117 and 119 providesa single low-intermediate-frequency signal at the output fordigitization by respective analog to digital converters. Alternateembodiments produce quadrature (I, Q) baseband signals for digitization.

The outputs of the transceiver are input to the receive part of atwo-input receive part of a digital modem 121 that includes for eachinput an analog-to-digital converter (ADC), a downconverter includingany filtering to produce baseband samples, an initial carrier frequencydetector to determine timing, and a discrete Fourier transformer (DFT)to convert the samples to tones. One embodiment includes a channelestimator for each received signal that determines the channelexperienced by each tone so far. The channel estimates are denoted H₁(k)and H₂(k) for the k'th tone, and first and second antenna paths via thefirst and second antennas 113 and 115, respectively, where H₁(k) andH₂(k) are complex valued quantities, e.g., have amplitude and phase inpolar coordinates, and quadrature (I, Q) components in rectangularcoordinates. One embodiment further includes a weight calculator thatdetermines how to combine the tones using the estimated channels foreach tone for each signal, and a beamformer that uses the calculatedweights to form combined tones for further processing. The furtherprocessing includes pilot correction using known pilot subcarriersincluded with an OFDM signal according to the standard. The pilotcorrected signals are then demodulated, and the demodulated signals arede-interleaved and de-punctured to form coded digital signals. The codeddigital signals are decoded to produce the digital information for a MACcontroller (not shown) for the AP.

Different criteria are used in different versions for calculating thebeamforming weights in the receive part of the digital modem 121. In oneembodiment, maximum ratio combining is used on a tone-by-tone basis. Inanother embodiment, for each tone, a “use one or the other antenna”decision is made to use the tone received from one antenna or the tonereceived from the other antenna based on comparing the channel responseamplitudes for the particular tone for the two antennas. This isequivalent beamforming using weights of either 0 or 1 on a scale of 0 to1 for each subcarrier.

Referring now to FIG. 1B, the access point includes a transmit part ofthe digital modem 121 that accepts digital information for transmissionfrom the AP's MAC controller (not shown), and carries out digitalmodulation tasks according to the standard, including scrambling to formscrambled information, encoding to encode the information, puncturingand interleaving to form interleaved coded information, and modulatingto form modulated OFDM tones. The modulated OFDM tones are now subjectedto transmit beamforming according to weights denoted W_(Tx1)(k) andW_(Tx,2)(k) for the k'th tone, for the first and second antennas 113 and115, respectively, to form OFDM tones transmission by each of theantennas 113 and 115. Each set of OFDM tones is subject to a inversediscrete Fourier transform (IDFT) operation and are cyclically extendedto form the OFDM modulated digital signal to which a preamble is addedto form the OFDM digital samples for each antenna. The digital samplesfor each antenna are converted to analog information viadigital-to-analog converters (DAC) to form the OFDM signals fortransmission by each antenna. Each analog signal is input to arespective transmitter RF part of two RF transceivers 117 and 119respectively coupled to each antenna 113 and 115. The RF signal is thustransmitted to the client 103.

The client receives the signal at its antenna 111 coupled to thetransmit part of the transceiver 109. The received signal is convertedto digital samples and processed in the receive part of the modem 107.

One aspect of the invention is that the client when receiving need nothave multiple antennas to benefit from the transmit beamforming at theAP. Another aspect of the invention is that the beamforming iscalibrated at the AP independent of the receive characteristics of theclient 103.

In one embodiment, the weight calculator in the AP's digital modem 121further uses the channel estimates determined by receiving from theclient 103 to determine beamforming weights to use for beamforming whenthe AP 105 transmits to the same client 103. Thus, the digital modem 121includes a memory 123 for storing the latest weight information, e.g.,in the form of the latest channel response information, for a number,e.g., any associated clients plus any others such that the memorycontains up to a predefined number of clients from which the AP mostrecently received information.

In one embodiment, for each tone, a “use one or the other antenna”decision is made to use one antenna or the other antenna fortransmitting based on comparing the channel response amplitudes for theparticular tone for the two antennas calculated from the most recentlyreceived packet from client 103. This is equivalent to using transmitweights of 0 or 1 in a range of 0 to 1. In another embodiment, amodified tone-by-tone maximum ratio combining criterion is used based onthe channel responses for the particular tone for the two antennascalculated from the most recently received packet from client 103.

The embodiments described in FIGS. 1A and 1B require a radio receiver,including much of the receive part of the digital modem for each receiveantenna for uplink communication, and a radio transmitter, including asection of the transmit path of the digital modem for each transmitantenna.

A much more economical approach is to have a single transceiver that foruplink communication can alternately connect to each of the two antennasduring the start of the packet and select the antenna based on somedecision metric. This approach is referred to as selection diversity.

FIGS. 2A and 2B show a simplified WLAN that includes a client and anaccess point that implements an alternate embodiment using selectiondiversity, so it does not require the same amount of electronics at theAP 205 as in the access point 105, and thus is a lower cost solutionthan that described above with reference to FIGS. 1A and 1B. Inparticular, the access point 205 includes a first antenna 213 and asecond antenna 215, both coupled to an antenna selector 217 that selectseither antenna 213 or antenna 215. The selector is coupled to both thereceive and transmit parts of a transceiver 219 via a duplexer that is aswitch in this embodiment. The transceiver is coupled to a digital modem221.

On the uplink, referring to FIG. 2A, the AP duplexer—a switch—connectsthe output of the antenna selector to the receive part of the RF radio219. A signal is transmitted by the client as described above and isreceived at both antennas. Prior art selection diversity receiversselect the antenna to use for reception based on such measures as theRSSI. In one embodiment, the access point operates as described inabove-mentioned incorporated-by-reference U.S. patent application Ser.No. 10/698,588 titled ERROR VECTOR MAGNITUDE SELECTION DIVERSITY METRICFOR OFDM. A packet conforming to the OFDM variants of the IEEE 802.11standard includes a preamble and a modulated part. The receive part ofthe modem 221 includes an EVM calculator 231 that calculates an errorvector magnitude (EVM) measure from a preamble part of a packet duringreception of the preamble. During such reception, the EVM measure isobtained via the first antenna 213 then via the second antenna 215. Inone embodiment, the EVM measure is obtained prior to automatic gaincontrol (AGC) so that AGC is performed on the antenna receiving theremainder of the packet. The EVM calculator 231 calculates a measure ofthe pre-AGC EVMs of the signals received via antenna 213 and 215,compares the calculated EVMs, and outputs an antenna select signalaccording to the superior metric. Thus, the EVM calculator 231 selectsvia a connection between the modem 221 and the selector 217 the antennathat gives the best EVM measure. The selector now connects such antennato the receive chain, and the remainder of the packet is received viathe selected antenna. The receive paths of the RF receiver 219 anddigital modem 221 thus receive and demodulate/decode the remainer of thepacket. Note that other than the selctor, EVM calculator, and associatedcontrol, only a single receive path is required. Thus, for receiving,the access point 205 is less expensive to manufacture than one such asAP 105.

FIG. 2B shows a simplified diagram of the downlink communication fromthe AP 205 to the same client station 103. A memory 223 stores theresults of comparing the EVM measure from both antennas when receivingfrom the client 103. The memory 223 stores EVM-based decisions for anumber of recently communicated—with client stations, e.g., the clientstations associated with the AP. The information in memory 223 for theclient station 103 is used to control the antenna selector 217 to selectone of the antennas 213, 215 for transmitting to the client station 103.Once the antenna for transmitting is selected, the packet fortransmission is encoded and modulated, then OFDM signal samples aregenerated. The OFDM signal samples are converted to analog signals andtransmitted as RF via the transmit part of the transceiver 219 and theselected transmit antenna.

Thus selecting the transmit antenna is an improvement over prior artRSSI-based selection criteria.

Compared to the transmitting shown in FIG. 1B, the transmittingdescribed in FIG. 2B does not require two RF transmit paths in thetransceiver, nor two IDFT operations in the transmit part of the modem.Thus, for transmitting to the access point, 205 is less expensive tomanufacture than one such as AP 105.

FIGS. 3A and 3B show the uplink and downlink communications of anotheralternate embodiment. An AP 305 communicating with the client 103includes a transceiver that has two RF receive paths, shown as RFreceivers 317 and 319 coupled to a first antenna 313 and a secondantenna 315, but a single transmit path including an RF radiotransmitter 325. The RF receivers and the RF transmitter are coupled toa digital modem 321. Referring to FIG. 3A, for uplink communication, thereceive part of the digital modem 321 includes two paths that eachincludes an EVM calculator for each antenna signal, shown as EVM1 331and EVM2 333 that each calculate a measure of the EVM based not on thepreamble, but rather on demodulated symbols of tones of the OFDM signalsfrom each antenna and respective RF receiver. In one embodiment, the EVMcalculation is as described in U.S. patent application Ser. No.10/367,010 to Ryan et al., filed Feb. 14, 2003 and titled SELECTING THEDATA RATE OF A WIRELESS NETWORK LINK ACCORDING TO A MEASURE OF ERRORVECTOR MAGNITUDE, Docket No. CISCO-6489. U.S. patent application Ser.No. 10/367,010 is incorporated herein by reference. A packet conformingto the OFDM variants of the IEEE 802.11 standard includes a low-ratecoded field called the SIGNAL field that describes how the remainder ofthe packet is encoded, e.g., the data rate and modulation. U.S. patentapplication Ser. No. 10/367,010 describes how the EVM of the SIGNALfield can be used to select the data rate for communication with aparticular station. In the case of the system of FIG. 3A, the EVM isused to determine the data rate for communicating with the client 103.One embodiment of the the receive part of the digital modem furtherincludes a beamforming weight calculatr as descibed with reference toFIG. 1A.

One aspect of the invention is that for transmitting to a clientstation, the EVM measures of signals received from the client througheach antenna are compared and are used for downlink comunication withthat client. Referring now to FIG. 3B, one embodiment of the digitalmodem 321 has a transmit section that is similar to that shown in FIG.2B, in that the modem 321 includes a memory 323 maintaining the EVMs, ordecisions based thereon, for communicating with recently communicatedwith, e.g., associated client stations. The transmit part path of theaccess point 305 includes a transmit antenna selector 307 coupled to thememory that selects the transmit antenna according to the stored EVMmeasures, or decisions therefrom. The modem includes only a singletransmit signal path, and the RF section includes only a single RFtransmitter 325 beween the to-be-transmitted signal output of thedigital modem and the antenna selector 327.

Note that using a single signal path for transmit is useful, even in thecase that two receive paths are used for receiving, e.g., as shown inFIGS. 3A and 3B, because a RF transmit path includes a transmit poweramplifier, so saving one RF transmit path is worthwhile.

FIG. 4 is a block diagram of a complete access point such as AP 105shown in more detail than the block diagrams of FIGS. 1A and 1B. Theaccess point includes a first antenna 113 and a second antenna 115coupled via a duplexing switch (not shown) to a first radio transceiver117 and a second radio transceiver 119. The first and second radiotransceivers 117, 119 include a radio receiver 413, 415, respectively,and a radio transmitter 417, 419, respectively. The first and secondradio transceivers are coupled to respective digital circuits 421 and423. Each digital circuit 421, 423 respectively includes a receivedigital path 425, 427 and a transmit digital path 429, 431. Each receivedigital path 425, 427 accepts low IF signals from the respective radioreceiver 413, 415 and digitizes the signals using a respective ADC 433,435. The digital samples from the respective ADC 433, 435 are acceptedby a respective start of packet (SOP) and automatic gain control (AGC)subsystem 437, 439. The respective radio receiver 413, 415 also providesa RSSI signal to the respective digital receive paths 425, 427, and theRSSI signal from the respective radio receiver 413, 415 is digitized bya respective RSSI ADC to provide RSSI signal samples to the respectiveSOP and AGC subsystem 437, 439. Each respective SOP and AGC subsystem437, 439 determines the start of packet, and also sets the gains of eachradio receiver 413, 415 via a gain control interface (GCI).

The digital samples from each respective ADC 433, 435 are downconvertedto produce baseband samples using a downconverter 441, 443 in eachdigital receive path 425, 427. The downconverted signals are convertedto modulated tones by a fast Fourier transform (FFT) unit 445, 447. Eachpacket conforming to an OFDM variant of the IEEE 802.11 standardincludes symbols of known subcarriers in the preamble. Each digitalreceive path 425, 427 includes a channel estimator 449, 451 acceptingthe output of the respective FFT unit 445, 447 during reception of theknown symbols and determines the channel response for each tone for eachantenna's receive path so far.

The access point also includes a beamforming subsystem 453 that forreception includes a weight calculator 455 that accepts the respectiveoutputs of the channel estimators 449, 451 for each antennas' receiver.The weight calculator in one embodiment calculates complex valuedreceive weights that are accepted by a receive beamformer 457. Thereceive beamformer 457 accepts the outputs of the respective FFT units445, 447 and forms a weighted signal for demodulation and decoding. Ademodulator (demod.) and decoder subsystem 459 carries out thedemodulation, de-interleaving, de-puncturing and de-scrambling to formthe digital data for a received packet. The output of the demodulatorand decoder subsystem 459 is accepted by a MAC processor 461.

Not shown in FIG. 4 are the timing and synchronizing units thatdetermine the timing, e.g., for the FFT units 445, 447. In oneembodiment, one of the digital receive paths, e.g., digital receive path425 acts as a master to the second digital receive chain 427 in that thetiming in the two digital receive paths are synchronized, with thedigital receive path 425 determining the timing for both.

One embodiment of the weight calculator 455 uses a maximum ratiocombining method to determine complex valued weights for the receivebeamformer 457 as described further below.

Another embodiment of the combination of the weight calculator andbeamformer examines the magnitude of the channel responses for the firstand second receive paths, and for each tone, selects the antenna paththat has the greater magnitude channel response. Thus, the demodulatorand decoder subsystem accepts for each tone the signal for demodulationfrom the receive path system that provided the “better” channel in termsof channel response magnitude. At any time, for any subcarrier eitherone or the other antenna's signal is used for demodulation and decoding.This is equivalent to using real-valued weights of 0 or 1 on a scale of0 to 1.

On the downlink, information from the MAC is accepted by a coder andmodulator 463 that scrambles and encodes the data, punctures andinterleaves the coded data, and modulates the data to form modulatedsymbols for each tone of a to-be-transmitted OFDM signal. Pilot tonesare combined to form a complete set of tones. The complete set of tonesare accepted by a transmit beamformer 465 that also accepts transmitweights from the weight calculator 455 to generate two tone-sets, onefor each transmit-chain to be transmitted by each of antennas 113 and115. The weights are from a memory 475, shown here as in the weightcalculator, and in general is coupled to the weight calculator.

One embodiment of transmit beamforming in the transmit beamformeraccepts complex valued transmit weights according to a modified maximumratio combining criterion. Such weights are obtained from the channelresponses of the last received packet. Another embodiment transmits eachtone either via the first or the second antenna depending on acomparison of the amplitude of the respective channel responses. This isequivalent to transmit beamforming using real valued transmit weights ofeither 0 or 1 on a scale of 0 to 1, although the implementation does notuse such weighting but rather a binary decision branch. How the transmitweights are calculated and other implementation aspects are described inmore detail below.

The two tone-sets from the transmit beamformer 465 are input to thefirst and second digital transmit paths 429, 431. Each digital transmitpath 429, 431 includes a respective inverse FFT (IFFT) unit 467, 469 toconvert the tone sets to time-domain to-be-transmitted digital signals.Each digital transmit path 429, 431 includes a mechanism (not shown inthis drawing) to add cyclic extension to the data corresponding to eachOFDM signal and a mechanism, also not shown in FIG. 4 to form arespective packet by adding a preamble to the respective data. Thecomplete digital data for each packet is converted to analog data by arespective DAC 471, 473 to generated I,Q data for the respective radiotransmitter 417, 419.

The respective radio transmitter 417, 419 transmits the packet via thefirst antenna 113 and second antenna 115, respectively. Not shown inFIG. 4 is the duplexer—a switch—for switching each of the antennasbetween the respective radio transmitter and the respective radioreceiver.

FIG. 5 is a block diagram illustrating in more detail than shown in FIG.4 one embodiment of the coding and modulation unit 463, and of the posttransmit beamformer processing. The data from the MAC processor 461,labeled mac_data, is in parallel form and is converted to a serialstream by a parallel-to-serial converter 503. A scrambler 505 acceptsand scrambles the data. A selector 507 selects the scrambled data or theunscrambled version according to a signal labeled scram. Assumescrambled data is output by selector 507. A convolutional encoder 509encodes the data. A selector 511 selects the coded data or the uncodedversion according to a signal labeled tx_coded. Assume coded data isoutput by the selector 511. The coded data is punctured by a puncturer513 and interleaved by an interleaver 515 to produce the interleaveddata for modulation. A modulator 517 modulates the data to generatemodulated symbols for each tone. Some of the tones are pilot tones, anda selector 521 selects whether the tone is a modulated symbol from themodulator 517 or a pilot tone from an included pilot tone generator 519that generated pilot tones. The tones are shaped by a shaper 525 toproduce the tones for multiplexing and transmission.

Note that some of the units in the coder and modulator 463 operateaccording to the data rate. Furthermore, the SIGNAL field specifying thedata rate and modulated with BPSK is first generated.

In one embodiment, the modulated signals are weighted by a weightingunit 465 that accepts transmit weights from the memory 475, shown aspart of the weight calculator 455. The output of the weight generator isthe two weighted tone symbols that are respectively input into IFFTunits 467 and 469 for multiplexing into OFDM signal samples fortransmission.

In another embodiment, the unit 465 implements tone-by-tone diversityselection according to a comparison of the magnitude of the estimatedchannel response for each tone via each of the antenna paths. In oneversion, the unit 455 is thus an antenna selector to select an antennatone-by-tone. Unit 465 thus switches the output of the coder andmodulator 463, e.g., the output of the shaper 525, between the input ofthe IFFT unit 467 and the input of IFFT unit 469. This, as describedabove, is equivalent to weighting by real-valued binary valued weightsof 0 or 1 on a scale of 0 to 1. Therefore, the output of unit 465 may becalled the weighted outputs in either the tone-by-tone diversity ortone-by-tone weighting embodiments.

The IFFT units 467, 469 produce the OFDM signals and are accepted bycyclic extension and windowing units 527, 529 to add a cyclic extensionand window each symbol. A preamble generator 531 produces the preamblefor each packet, and has an output that is scaled by a scaler 533. Foreach transmit path, a respective selector 535, 537 initially selects thescaled preamble generated by the preamble generator 531 and the scaler533 according to a signal called Preamble Enable. The Preamble Enableswitches the respective selector 535, 537 to accept the cyclicallyextended OFDM signals to produce I,Q samples for conversion to analogI,Q signals by respective DAC 471, 473 for transmission by the antenna113, 115, respectively.

EVM-Based Selection Diversity Transmission

As described above with reference to FIGS. 2A and 2B, one embodimentincludes selecting the antenna according to a measure of the EVM of apreamble part received via both antennas.

By “a measure of the relative EVM” in general is meant any measure thatvaries monotonically with an approximation of the EVM, e.g., with anapproximation of the measure of the RMS distance between receivedsymbols and ideal symbols, divided by the RMS distance from idealsymbols to zero. Note that in this description, the averaging is carriedout after division. In alternate embodiments, the averaging is carriedout prior to division. As will be shown later, several methods arepresented for determining a measure of the relative EVM, e.g., as anapproximation to the relative EVM.

The most accurate EVM estimate would require demodulating the packet andcomputing the EVM directly by comparing the measured symbol positions tothe ideal symbol positions. In the embodiment of FIG. 2A, the EVM unit231 measures and compares a measure of the EVM and carries out antennadiversity selection prior to AGC so that the gain can afterwards be setappropriately for the selected antenna by the AGC method. In oneembodiment, AGC also takes place before the end of the short preamblepart to allow enough time for other necessary radio functions to occur.In one embodiment, the AGC method is as described in U.S. patentapplication Ser. No. 10/622,175 to Adams et al. filed Jul. 17, 2003 andtitled ADAPTIVE AGC IN A WIRELESS NETWORK RECEIVER, Attorney/AgentDocket No. CISCO-7343, and includes setting the gains in a set ofstages. When antenna diversity selection is included, selecting theantenna from a set of antennas replaces the first AGC stage, and occursduring the short preamble period and while the gains are set to adefault gain level. One embodiment of the selection method takes placeover two short sequence times, one short sequence period per antenna,after SOP detection.

The antenna selection also is carried out prior to initial timingestimation that determines the timing of the short symbols. Thus, atthis early stage in the short preamble period, the inventors chose touse an EVM calculator that approximates the EVM without requiring thatthe short symbol timing be determined and without demodulating. Ofcourse alternate embodiments may use different methods for calculatingthe EVM (see FIG. 3A).

In calculating a measure approximating the EVM without requiringaccurate timing, an assumption is made that the EVM is due only to noiseor colored interference; other EVM contributors are neglected. Ofcourse, the method operates even if such other sources of error exist.The EVM calculator is simply less accurate under such conditions.Experiments demonstrated, however, that the antenna selection methodworks reasonably well even with this approximate measure of the EVM.

Under this assumption, in one embodiment, an approximate measure of therelative EVM is determined by determining the symbol vector magnitude(SVM) during the short sequences and the noise power per subcarrierprior to the short sequences.

Note that in practice, noise samples from only one of the two antennasare taken.

According to the IEEE 802.11 standard, only 12 out of the 52 subcarriersare used in the short sequences. In one embodiment, the SVMs for eachantenna, e.g., antenna 1 are determined by gathering one-short symbol'sworth of consecutive samples, i.e., 16 consecutive short sequencesamples when sampling at 20 MHz, from antenna 1, x₁[i] for 0≦i≦15, andperforming a discrete Fourier transform (DFT) on these samples.Specifically, in one embodiment, the symbol vector magnitudes persubcarrier are estimated by $\begin{matrix}{{{{SVM}_{1}\lbrack k\rbrack} = {\sqrt{\frac{3}{13}}\frac{1}{16}{{\sum\limits_{i = 0}^{15}\quad{{x_{1}\lbrack i\rbrack}{\exp\left( \frac{{- {j2\pi}}\quad{ki}}{16} \right)}}}}}},} & {{Eq}.\quad 1}\end{matrix}$

-   -   for k=1, 2, 3, 4, 5, 6, 10, 11, 12, 13, 14, and 15. Only these        twelve SVMs can be estimated during the short sequences because        only 12 out of the 52 subcarriers are used in the short        sequences. As a consequence of only 12 subcarriers being used in        the short sequences, these 12 subcarriers have 13/3 times the        power of the packet subcarriers to maintain constant signal        power between the short sequences and the remainder of the        packet when all 52 subcarriers are used. The factor √{square        root over (3/13)} in the above equation is used to adjust        between the difference in the short sequence subcarrier power        and packet subcarrier power. This is because the SVMs are        assumed to be equal to the square root of the packet subcarrier        powers.

The next step in calculating the relative EVM is to determine thesubcarrier noise power. In one embodiment, it is assumed measurementsfrom a first antenna—denoted antenna 1 here—is available.

In a first variation, the determination of a measure of the relative EVMdoes not require a determination of the noise, in the sense that anassumption is made that the noise is additive white noise and the samenoise power appears at each receive antenna, i.e., that the noise is thesame for each subcarrier and for each antenna. Thus, according to thefirst variation, the selection is made according to a measure:$\begin{matrix}{{{{Relative}\quad{EVM}_{m}} = {\sum\limits_{k}\quad\frac{1}{{SVM}_{m}\lbrack k\rbrack}}},} & {{Eq}.\quad 1}\end{matrix}$

-   -   where m=1 or 2, indicating the first or second antenna. This        variation has an advantage that it is easy to implement. For        example, a lot of the scale factors, e.g., sqrt(3/13), and many        of the terms for the noise power become unimportant.

Another variation uses an estimate of the power spectral density of thenoise from antenna 1, as well as a power spectral density of the signal.Thus, one implementation of this method includes:

-   -   (a) Determining a 16 point-FFT on the baseband noise in antenna        1 sometime before the start of packet while the receiver is set        to its default gain settings. This provides a measure of the        power spectral density of the noise. It is assumed that the        power spectral density of the noise from antenna 1 applies also        to antenna 2.    -   (b) Determining a 16 point FFT on the known second short symbol        to determine a measure of the power spectral density of the        signal using the 12 short symbol subcarriers.    -   (c) Assuming an upper bound on the baseband SNR, the post AGC        subcarrier to noise ratio (SCNR) is estimated for each of the 12        short symbol subcarriers.    -   (d) Computing the relative EVM.

(e) Selecting the receive antenna with the lowest estimated error.

Furthermore, in the above-described embodiments, the relative. EVMdeterminations, according to any of the variations, occur pre-AGC. Inyet another set of variations, the determining of the measure of therelative EVM occur with signals post-AGC. For example, this may occur bycarrying out AGC on a first antenna, obtaining the information needed todetermine a measure of the relative EVM on the first antenna, carryingout AGC on the other, second antenna, then obtaining the informationneeded to determine a measure of the relative EVM on the second antenna.With this set of variations, the relative EVMs of the two antennas areobtained post-AGC, and compared to select the one antenna or the other.

The Receive Weight Calculation Method

The receive weight calculation carried out by one embodiment of weightcalculator 475 is now described. The weight calculator 475 processesdata after the FFT. Therefore the subsequent notation is complex-valuedand in the frequency domain, e.g., for each tone, whether or not thetone dependency is explicitly shown. Denote the tones of a transmitteddata stream by Z(k), where k=−26, −25, −24, . . . , −1, 1, . . . , 25.26 denote the 52 frequency tones according to The OFDM variants of theIEEE 802.11 standard. Suppose in general there are L antennas. L=2 inthe above drawings. Denote by Y₁(k), . . . , Y_(L)(k) the receivedsignals on branch 1, . . . L, respectively. Denote by H₁(k), . . . ,H_(L)(k) the channel experienced by tone k for receive paths 1 through Lcorresponding to antennas 1, . . . , L respectively. The linear systemmodel is given as follows ${\begin{bmatrix}{Y_{1}(k)} \\\vdots \\{Y_{L}(k)}\end{bmatrix} = {{\begin{bmatrix}{H_{1}(k)} \\\vdots \\{H_{L}(k)}\end{bmatrix}{Z(k)}} + \begin{bmatrix}{N_{1}(k)} \\\vdots \\{N_{L}(k)}\end{bmatrix}}},{or}$ Y(k) = H(k)Z(k) + N(k),

where the boldface denotes vector quantities, N₁(k), . . . , N_(L)(k)denoted the noise, assumed additive white Gaussian noise (AWGN) onreceive paths 1 through L, for tone k.

The received signal is processed by the receive beamformer 547 togenerate the estimate denoted {circumflex over (Z)}(k) of thetransmitted data streams for tone k, with${{\hat{Z}(k)} = {\left\lbrack {{W_{R1}(k)}\quad\cdots\quad{W_{RL}(k)}} \right\rbrack\begin{bmatrix}{Y_{1}(k)} \\\vdots \\{Y_{L}(k)}\end{bmatrix}}},{or}$ Ẑ(k) = W_(R)(k)Y(k)

where the receive beamformer weights for tone k are denoted by W_(R1)(k), . . . W_(RL) (k) for paths 1, . . . , L. In one embodiment, thebeamforming step of beamformer 455 is performed for all 52 non-zerotones for every OFDM symbol.

For reception, in one embodiment, antenna combining weighting is givenbelow W_(R)(k) = [Ĥ^(*T)(k)R⁻¹(k)Ĥ(k)]⁻¹Ĥ^(*T)(k)R⁻¹(k)${R(k)} = \begin{bmatrix}{\sigma_{1}^{2}(k)} & \cdots & {\sigma_{1L}^{2}(k)} \\\vdots & ⋰ & \vdots \\{\sigma_{L1}^{2}(k)} & \cdots & {\sigma_{L}^{2}(k)}\end{bmatrix}$ σ_(ij)²(k) = E{v_(i)(k)v_(j)^(*)(k)}v_(i)(k) = Y_(i)(k) − Ĥ_(i)(k)Ẑ_(Hard)(k)

-   -   where( )* denotes the complex conjugate, ( )^(T) denotes the        matrix transpose, Ĥ(k) is the L-vector estimate of the receive        channel on tone k, and R(k) is the noise and interference        covariance matrix for tone k. {circumflex over (Z)}_(Hard) (k)        is the hard decision—the nearest constellation point—of the        estimate of the transmitted data stream.        Channel Estimation

In one embodiment, the receive weights, and consequently the transmitweights are calculated using the channel estimates made by channelestimators 449 and 451. In such an embodiment the channel estimation iscarried out as described in U.S. patent application Ser. No. 10/217,117titled CHANNEL ESTIMATION IN A MULTICARRIER RADIO RECEIVER, filed Aug.12, 2002, Docket/Reference No. CISCO-5748. U.S. patent application Ser.No. 10/217,117 is incorporated herein by reference. The preamble of eachpacket conforming to the OFDM variants of the IEEE 802.11 standardincludes two symbols—the “long symbols”—that have known tones. Each ofchannel estimators 449 and 451 receives FFT data for the two longsymbols during the long symbol period and computes the channel response,denoted H₁(k) and H₂(k). The channel estimates are denoted H, (k) and H₂(k) for data received via the first and second antennas 113 and 115.Each estimate is obtained as the average over the two long symbols, andcan be expressed as follows${{\hat{H}}_{1}(k)} = \frac{{Y_{1}\left( {k,1} \right)} + {Y_{1}\left( {k,2} \right)}}{2 \cdot {{LS}(k)}}$${{\hat{H}}_{2}(k)} = \frac{{Y_{2}\left( {k,1} \right)} + {Y_{2}\left( {k,2} \right)}}{2 \cdot {{LS}(k)}}$

-   -   where Y₁(k,1) is the FFT data from the FFT unit 445 of the        digital receive path 425 during the first long training symbol,        Y₁(k,2) is the FFT data from the FFT unit 445 of the digital        receive path 425 during the second long training symbol, Y₂(k,1)        is the FIT data from the FFT unit 445 of the digital receive        path 425 during the first long training symbol, and Y₂(k,2) is        the FFT data from the FFT unit 445 of the digital receive path        425 during the second long training symbol. LS(k) denotes the        known long symbol data at tone k. The channel estimation step is        performed for all 52 non-zero tones.

As described in U.S. patent application Ser. No. 10/217,117, the channeldetermination carried out by each of the channel estimators 449, 451includes tone smoothing. With tone smoothing, a channel estimate at tonek is averaged with its adjacent neighbors to further reduce the noise inthe channel estimate to take into account any correlation betweenadjacent tone's channel responses. The smoothed channel estimates aredenoted as {tilde over (H)}_(i)(k) and {tilde over (H)}₂(k) for antennas113 and 115 for tone k. In one embodiment, the smoothing is of each ofthe closest neighbors, such that $\begin{matrix}{{{{\overset{\sim}{H}}_{i}(k)}} = \frac{\sum\limits_{m = {- 1}}^{1}\quad{a_{m} \cdot {{{\hat{H}}_{i}\left( {k - m} \right)}}}}{\sum\limits_{m = {- 1}}^{1}\quad a_{m}}} & {{{- 25} \leq k \leq {- 2}},{2 \leq k \leq 25}} \\{{{angle}\left\{ {{\overset{\sim}{H}}_{i}(k)} \right\}} = \frac{\sum\limits_{m = {- 1}}^{1}\quad{{a_{m} \cdot {unw\_ ang}}\left\{ {{\hat{H}}_{i}\left( {k - m} \right)} \right\}}}{\sum\limits_{m = {- 1}}^{1}\quad a_{m}}} & {{{- 25} \leq {k - 2}},{2 \leq k \leq 25}} \\{{{\overset{\sim}{H}}_{i}(k)} = {{{\hat{H}}_{i}(k)}\quad k}} & {{k = {- 26}},{- 1},1,26}\end{matrix}$

Note that in the calculation of angle {{tilde over (H)}_(i)(k)}, thatthe angles of Ĥ_(i) (k) must be unwrapped prior to the calculation. Thisis denoted by the function “unw_ang”. The magnitude function is denotedby| |. The tone smoothing weights are denoted by am. With a frequencyselective channel, the weights are set a_(−1, =1), a₀=2, and a₊₁=1 so asto not smooth the frequency response as much. The filter length can befixed at 3-taps. The tap values may be selectable. In order not to delaythe decoding of the SIGNAL field, in one embodiment, a non-smoothedchannel estimate average is used during the SIGNAL interval.

Note that in one embodiment, the smoothing can be disabled. Furthermore,in one embodiment, the channel estimation includes channel tracking suchthat the channel estimate is updated as more data is decoded. Oneembodiment of channel tracking is described in U.S. patent applicationSer. No. 10/807,547 to Hart et al, filed Mar. 22, 2004, titled CHANNELTRACKING IN AN OFDM WIRELESS RECEIVER, Reference/Docket No. CISCO-7703.U.S. patent application Ser. No. 10/807,547 is incorporated herein bereference. Thus by the end of the reception of the packet, the channelestimates in channel estimators 449, 451 are updated. In one embodiment,the channel tracking method includes obtaining a first estimate of thechannel response for each tone, and accepting a pre-decisionconstellation point value for the tone. The pre-decision constellationpoint value is channel corrected using the first estimate of the channelresponse. The channel tracking method further includes making a decisionusing the pre-decision constellation point value, re-modulating thedecision to form a post-decision constellation point value, and forminga complex valued product of the function of the first estimate for thesubcarrier and the complex-valued ratio of the pre-decision andpost-decision values. This complex valued product forms the channeldrift to use for updating the stored channel response. In oneembodiment, the method includes updating the stored first estimate ofthe channel response with a weighted amount of the formed complex valuedproduct. In one embodiment, the first estimate of the channel responseis the smoothed channel estimate obtained as described above and in U.S.patent application Ser. No. 10/217,117.

Receive Weights Calculation

Different embodiments of the receiver's weight calculator 455 determinethe receive beamformer weights using different methods. One embodimentuses “power combining.” Power combining works well under the assumptionthat the spectral shapes of the noise in the signals received in the twosignal paths via the first and second antenna are similar. Thisassumption is approximated, for example, when the analog and digitalfiltering across the passband in each antenna's receive path is similar.Power combining works well, furthermore, under the additionalassumptions that: 1) the noise power is close in the two antenna paths,for example, if the noise figure of the two antenna signal paths areequal to within a few dB; and 2) the gain of the two antenna signalpaths are equal to within a few dB.

The power combining method includes using receive weights, denotedW_(Rx,1)(k) and W_(Rx,2)(k), for each of 52 tones k calculated asfollows${W_{{Rx},i}(k)} = \frac{{\overset{\sim}{H}}_{i}^{*}(k)}{{{{\overset{\sim}{H}}_{1}(k)}}^{2} + {{{\overset{\sim}{H}}_{2}(k)}}^{2}}$i = 1, 2

These receive beamformer weights are used in the beamformer for everyOFDM symbol in the packet: e.g., the SIGNAL field and the following datasymbols.

Another receive weight calculation method is maximum ratio combining(MRC). Tone-by-tone MRC works well when the noise in each of the twoantenna signal paths is spatially uncorrelated. This would be the case,for example, when there is no co-channel interference. The receiveweights W_(Rx,1)(k) and W_(Rx,2)(k) according to MRC are${W_{{Rx},i}(k)} = \frac{\frac{{\overset{\sim}{H}}_{i}^{*}(k)}{\sigma_{i}^{2}(k)}}{\frac{{{{\overset{\sim}{H}}_{1}(k)}}^{2}}{\sigma_{1}^{2}(k)} + \frac{{{{\overset{\sim}{H}}_{2}(k)}}^{2}}{\sigma_{2}^{2}(k)}}$

-   -   where σ_(i) ² (k) ≡noise variance estimate on signal path i and        tone k i=1,2

This uses an estimate of the noise. In a multipath fading environment,the average power level received on one antenna could be much differentthan on the other due to flat fading. Independent AGC circuits on eachreceive antenna branch may cause the perceived average received powerlevel on the two antennas to be equivalent. The primary goal of noiseestimation for MRC processing is correct for the AGC. A secondary reasonfor noise estimation is to account for noise figure differences on thetwo receive signal paths via the two antennas.

Various methods for noise estimation are possible. One embodimentincludes making noise estimates on the data and pilots tones andaveraging the initial estimates across the frequency band. In oneembodiment, the initial noise estimate on each antenna signal path,denoted v₁(k) and v₂ (k) for the first and second signal paths for eachtone k is calculated during the two long training symbols as follows${v_{1}(k)} = \frac{{Y_{1}\left( {k,1} \right)} - {Y_{1}\left( {k,2} \right)}}{\sqrt{2}}$${v_{2}(k)} = \frac{{Y_{2}\left( {k,1} \right)} - {Y_{2}\left( {k,2} \right)}}{\sqrt{2}}$

-   -   where Y₁(k,1) and Y₁(k,2) are the FFT data from first digital        path 425 during the first long symbol and second long training        symbol, respectively, Y₂(k,1) and Y₂(k,2) are the FFT data from        second digital path 427 during the first long symbol and second        long training symbol, respectively.

The noise power for each tone and antenna signal path is calculated asfollows:σ₁ ²(k)=v₁(k)v₁*(k)σ₂ ²(k)=v₂*(k)v₂*(k)

Initially, it is assumed that the noise is flat across the frequencyband of the signal. This assumption is based on digital and analogfilters being fairly flat across the passband. The noise is thenaveraged across all frequency tones, as follows${\sigma_{1}^{2}(k)} = {\frac{1}{52}{\sum\limits_{{m = {- 26}}{m \neq 0}}^{26}\quad{\sigma_{1}^{2}(m)}}}$${\sigma_{2}^{2}(k)} = {\frac{1}{52}{\sum\limits_{{m = {- 26}}{m \neq 0}}^{26}\quad{\sigma_{2}^{2}(m)}}}$

In an improved embodiment, to further refine the noise estimates,averaging is performed in time.

For each OFDM symbol, the receive beamformer 457 receives FFT data fromthe two digital receive paths 425, 427, denoted here by Y₁(k) and Y₂(k),respectively. The output of the beamformer gives the estimate of datastream, depicted by {circumflex over (Z)}(k). $\begin{matrix}{{\hat{Z}(k)} = \left\lbrack {W_{{Rx},1}(k)} \right.} & \left. {W_{{Rx},2}(k)} \right\rbrack\end{matrix}\begin{bmatrix}{Y_{1}(k)} \\{Y_{2}(k)}\end{bmatrix}$

All 52 non-zero tones for each receiver signal path are thus receivedbeamformed.

It is known that frequency and clock sampling offset causes the phasechannel to change from channel estimate as the packet progresses.According to the current (2004) OFDM variants of the IEEE 802.11standard, four tones are reserved as pilot tones for phase correction.These tones are number −21, −7, +7, +21 based on a −26 to +26 numberingscheme (with tone 0 being a zero tone). In one embodiment, thebeamformed received signals are pilot corrected in the demodulation anddecoding unit 459 using the pilot tones.

Transmit Weights Calculation

One aspect of the invention is transmit weight calculation based on theestimated channel responses. Another aspect of the invention is transmitbeamforming without requiring the receiving station, e.g., the client103 to have multiple antennas, and without requiring calibration at thereceiving client.

According to the OFDM variants the IEEE 802.11 standard, the receive andtransmit frequencies for communicating between two stations, e.g.,between client 103 and AP 105 occurs at the same frequencies. Thus, itis reasonable to assume reciprocity applies.

In the following formulae, the dependence on k, the tone, is left outfor simplicity. The equations, however, are in the frequency domain andapply to each tone k. Furthermore, the quantities are in general complexvalued, as would be clear to those in the art. Thus, a complex valuedquantity has an amplitude denoted by || and a phase, denoted by angle(). Referring again to FIG. 1A, denote by H_(R,i), i=1,2 the channelresponse experienced by the i'th, e.g., the first and second overallreceive paths via antenna i, i=1,2, e.g., via the first and secondantenna, respectively. The channel estimators 449, 451 respectivelyobtain an estimate of these channel responses. Each channel response ismade up of several components. Denote by H_(C,Tx) the channel responseof the transmit signal path of the client station 103. Denote by H_(i),i=1, 2 the channel response of the wireless link between the transmitantenna 111 of the client station 103 and the i'th antenna, i=1,2, i.e.,antenna 113, 115 of the AP 105. Further, denote by H_(AP,RF,Rx,i), 1=1,2 the channel response for each tone of the AP's analog receive signalpath via the i'th antenna, i=1,2, and denote by H_(AP,D,Rx), the channelresponse for each tone of the AP's digital receive signal path for eachof the antenna signals. As written, H_(AP,RF,Rx,i), i=1, 2 includes theresponse of any analog components in the receive signal paths, such thatany further processing is carried out in the digital domain. Because allprocessing, e.g., the filtering carried out in the downconversion isdigital, both signal paths through the first and second antennasexperience the same digital response H_(AP,D,Rx).

The overall receive channel responses areH _(R,1) =H _(C,Tx) ·H ₁ ·H _(AP,RF,Rx,1) ·H _(AP,D,Rx)H _(R,2) =H _(C,Tx) ·H ₂ ·H _(AP,RF,Rx,2) ·H _(AP,D,Rx)

Referring now to FIG. 1B, denote by W_(Tx,1)(k) and W_(Tx,2)(k) thetransmit beamformer weights used by beamformer 457 (FIGS. 4 and 5) fortransmitting from the AP 105 to the client station 103. Denote byH_(AP,D,Tx), the channel response for each tone of the AP's digitaltransmit signal path for to-be-transmitted the antenna signals, anddenote by H_(AP,RF,Tx,i), i=1, 2 the channel response for each tone ofthe AP's analog transmit signal path via the i'th antenna, i=1,2. Aswritten, H_(AP,RF,Tx,i)=1, 2 includes the response of any analogcomponents in the transmit signal paths. Because all prior processing iscarried out in the digital domain, the signal paths for transmission viathe first and second antennas, respectively, experience the same digitalresponse H_(AP,D,Rx). Assuming reciprocity, the signals from eachantenna 113, 115 to the client 103's antenna 111 are H₁ and H₂,respectively. Denote by H_(C,Rx) the receive signal path channelresponse at the client for each tone. Then the overall receive channelresponse experienced by a signal received at the client 103, withbeamforming, isH=W _(Tx,1) ·H _(AP,D,Tx) ·H _(AP,RFTx,1) ·H _(C,Rx) +W _(Tx,2) ·H_(AP,D,Tx) ·H _(AP.RF,Tx,2) ·H _(2·H) _(C,Rx)

Different embodiments set the transmit weights differently. Oneembodiment selects the transmit weights to be proportional to thecomplex conjugate of the estimated channel responses using the noisefree-version of MRC, which corresponds to the power combining method.That is${W_{{Tx},i}(k)} = \frac{{\overset{\sim}{H}}_{i}^{*}(k)}{{{{\overset{\sim}{H}}_{1}(k)}}^{2} + {{{\overset{\sim}{H}}_{2}(k)}}^{2}}$i = 1, 2

-   -   where {tilde over (H)}_(i)(k), i=1,2 are the channel estimates        based on the most recently received packets from the client        station 103.

The inventors found that using such weights can produce a largevariation in the antenna outputs. Therefore in another embodiment, amodified noise-free MRC method is used which selects only the phase ofthe estimated channel response based on the most recently receivedchannel. That is${{{{angle}\left( {W_{{Tx},i}(k)} \right)} = {- {{angle}\left( {{\overset{\sim}{H}}_{i}(k)} \right)}}};{{{W_{{Tx},i}(k)}} = {{1/2.}{i.e.}}}},{{W_{{Tx},i}(k)} = {{\frac{{\overset{\sim}{H}}_{i}^{*}(k)}{2{{{\overset{\sim}{H}}_{i}(k)}}}i} = 1}},2$

Then, substituting the channel contributions for the channel estimates,the overall channel experienced by each tone received at the client is$H = {\frac{H_{R,1}^{*} \cdot H_{{AP},D,{Tx}} \cdot H_{{AP},{RF},{Tx},1} \cdot H_{1} \cdot H_{C,{Rx}}}{2{H_{R,1}}} + \frac{H_{R,2}^{*} \cdot H_{{AP},D,{Tx}} \cdot H_{{AP},{RF},{Tx},2} \cdot H_{2} \cdot H_{C,{Rx}}}{2{H_{R,2}}}}$

This may be re-written as$H = {\frac{1}{2}{\left( {H_{{AP},D,{Tx}} \cdot H_{{AP},{RF},{Tx},1} \cdot H_{C,{Rx}}} \right) \cdot {\left( {\frac{H_{C,{Tx}}^{*}}{H_{C,{Tx}}} \cdot \frac{H_{{AP},D,{Rx}}^{*}}{H_{{AP},D,{Rx}}} \cdot \frac{H_{{AP},{RF},{Rx},1}^{*}}{H_{{AP},{RF},{Rx},1}}} \right)\left\lbrack {{H_{1}} + {\frac{H_{{AP},{RF},{Tx},2} \cdot \frac{H_{{AP},{RF},{Rx},2}^{*}}{H_{{AP},{RF},{Rx},2}}}{H_{{AP},{RF},{Tx},1} \cdot \frac{H_{{AP},{RF},{Rx},1}^{*}}{H_{{AP},{RF},{Rx},1}}} \cdot {H_{2}}}} \right\rbrack}}}$

FIG. 6 shows the above formula and the three major contributors. Thefirst contribution 601 is(H _(AP,D,Tx) ·H _(AP,RF,Tx,1) ·H _(C,Rx))

The terms here include any transmit digital filtering and transmit RFfiltering in the AP 105, and any receive digital filtering and receiveRF filtering in the client 103. The effects, however, are similar to thecase of a single antenna system in the AP. Note that the magnitudes ofany receive digital filtering and receive RF filtering in the AP 105,and any transmit digital filtering and transmit RF filtering in theclient 103 do not contribute to this term.

The second term 602 is a phase term that includes the angles of threetransfer functions, and contributes the following angle to overall phase

-   -   -angle(H_(C,Tx))-angle(H_(AP,D,Rx))-angle(H_(AP,RF,Rx,1)).

Thus, the phase of any receive digital filtering and receive RFfiltering in the AP 105, and any transmit digital filtering and transmitRF filtering the client 103 do have an effect compared to using a singleantenna on transmit from the AP.

The third term includes the factor 603 and is$\left\lbrack {{H_{1}} + {\frac{H_{{AP},{RF},{Tx},2} \cdot \frac{H_{{AP},{RF},{Rx},2}^{*}}{H_{{AP},{RF},{Rx},2}}}{H_{{AP},{RF},{Tx},1} \cdot \frac{H_{{AP},{RF},{Rx},1}^{*}}{H_{{AP},{RF},{Rx},1}}} \cdot {H_{2}}}} \right\rbrack.$

Is it desired that the real part of term 603, namely$\frac{H_{{AP},{RF},{Tx},2} \cdot \frac{H_{{AP},{RF},{Rx},2}^{*}}{H_{{AP},{RF},{Rx},2}}}{H_{{AP},{RF},{Tx},1} \cdot \frac{H_{{AP},{RF},{Rx},1}^{*}}{H_{{AP},{RF},{Rx},1}}}$is positive and relatively large with respect to the imaginary part ofterm 603, such that there is a positive contribution in the beamforming.The worse case is that${\frac{H_{{AP},{RF},{Tx},2} \cdot \frac{H_{{AP},{RF},{Rx},2}^{*}}{H_{{AP},{RF},{Rx},2}}}{H_{{AP},{RF},{Tx},1} \cdot \frac{H_{{AP},{RF},{Rx},1}^{*}}{H_{{AP},{RF},{Rx},1}}} = {- 1}},$

-   -   such that there is perfect cancellation of the signals.

Equal gain combining, which is only slightly inferior to MRC, is whenthis term 603 is +1, i.e.,${\frac{H_{{AP},{RF},{Tx},2} \cdot \frac{H_{{AP},{RF},{Rx},2}^{*}}{H_{{AP},{RF},{Rx},2}}}{H_{{AP},{RF},{Tx},1} \cdot \frac{H_{{AP},{RF},{Rx},1}^{*}}{H_{{AP},{RF},{Rx},1}}} = 1},$

-   -   such that the factor is (|H₁(k)|+|H₂(k)|).

One aspect of the invention is the matching of the magnitude and phaseof the transmit RF signal paths via each antenna in the AP, such thatH _(AP,RF,Tx,1) =H _(AP,RF,Tx,2)·

Another aspect of the invention is the matching of the phase of thereceive RF signal paths via each antenna in the AP, such that

-   -   angle(H_(AP,RF,Rx,1)(k))=angle(H_(AP,RF,Rx,2)(k)) for all tones        k.

Note that the client hardware has no effect on the third term.

One embodiment of the AP transceiver uses a superheterodynearchitecture. The transceiver is a single chip other than theintermediate frequency filters that are external SAW devices. For suchan architecture, in order to keep gain variations relatively low, oneembodiment uses high quality IF filters in the RF paths of the AP.

One embodiment of the access point is preferably constructed on a singleprinted circuit board (PCB). The RF transceivers 117, 119 and modems421, 423 are each implemented with CMOS technology in individualintegrated circuits (chips). The printed circuit boards are constructedsuch that the receive and transmit signal paths to each antenna arematched, e.g. by ensuring the same length of the etched signal traces,and the same neighboring signal traces. In one embodiment, the RFtransceivers use a superheterodyne architecture with external IF filers.In such an embodiment, the external transmit filters are matched.Furthermore, the external receive filters also are matched, at least inphase.

In an alternate embodiment, the elements such as the IF filers may ormay not be initially matched, but are provided along with a calibrationand correction procedure the effectively matches these components. Forinstance, one embodiment includes at manufacture time, measured andrecorded open loop calibration information e.g., as at least one table.The at least one table provides different calibration values fordifferent transit powers, different receive gains, different band and/orfrequency channels, or different temperature. Another embodimentincludes a provision for closed loop (in-service) calibration. Anysignals for transmission are separately adjusted by these calibrationvalues.

Thus, the matching may be carried out by configuration at manufacture,or after manufacture.

By so processing the signal, the transmitted signals are steered towardsthe receiving client 103, and furthermore, the transmitted signals arepre-equalized such that the client 103 has an easier receive signal toprocess than if no pre-equalizing occurred.

In one embodiment, the weight calculating and other processing iscarried out by a programmable processor.

Note that while the description herein is for implementation in an APfor communication with a client of the AP, the method is more generalfor implementation in a first wireless station for communication with asecond wireless station, the first station having a plurality ofantennas and a corresponding plurality of receive signal paths andtransmit signals paths, one transmit and one receive signal path perantenna. In one exemplary arrangement, the first station is a clientstation, and the second station is an AP.

While the description herein is for the first station having twoantennas and two each of a corresponding receive signal path andtransmit signal path, the invention is not restricted to two antennas,and may be generalized to a station with more than two antennas forreceiving and transmitting.

It should be appreciated that although the invention has been describedin the context of the OFDM variants of the IEEE 802.11 standard, theinvention is not limited to such contexts and may be utilized in variousother systems that use OFDM for receiving packet data. OFDM is oneexample of a multicarrier system in which the signal for transmission issplit into a set of subcarriers. The invention may also be applicable toother wireless receivers that use multicarriers.

While an embodiment has been described for operation in an OFDM receiverwith RF frequencies in the 2 GHz range (802.11 g) and 5 GHz range(802.11a), the invention may be embodied in receivers and transceiversoperating in other RF frequency ranges.

The IEEE 802.11a and 802.11g standards use OFDM and a preamble with twoidentical known long symbols that provide for channel estimation. Theinvention may be used with any data that includes known transmittedsymbols or transmitted signals that may be accurately determined at thereceiver. For example, the invention may include any number of knownsymbols at known locations. The symbols need not be identical.Furthermore, the symbols may be known because of the packet structure,or may become known via decision-direction and/or decoded-decisiondirection.

One embodiment of each of the methods described herein is in the form ofa computer program that executes on a processing system, e.g., one ormore processors that are part of an OFDM wireless receiver. The receiveand transmit digital signal paths in one embodiment include a processor,and for example, the weight calculator 455 is in one embodiment aprocessing system. Thus, memory 475 includes the memory of theprocessor.

Thus, as will be appreciated by those skilled in the art, embodiments ofthe present invention may be embodied as a method, an apparatus such asa special purpose apparatus, an apparatus such as a data processingsystem, or a carrier medium, e.g., a computer program product. Thecarrier medium carries one or more computer readable code segments forcontrolling a processing system to implement a method. Accordingly,aspects of the present invention may take the form of a method, anentirely hardware embodiment, an entirely software embodiment or anembodiment combining software and hardware aspects. Furthermore, thepresent invention may take the form of carrier medium (e.g., a computerprogram product on a computer-readable storage medium) carryingcomputer-readable program code segments embodied in the medium. Anysuitable computer readable medium may be used including a magneticstorage device such as a diskette or a hard disk, or an optical storagedevice such as a CD-ROM, or in the form of carrier wave signals.

It will be understood that the steps of methods discussed are performedin one embodiment by an appropriate processor (or processors) of aprocessing (i.e., computer) system executing instructions (codesegments) stored in storage. It will also be understood that theinvention is not limited to any particular implementation or programmingtechnique and that the invention may be implemented using anyappropriate technique for implementing the functionality describedherein. The invention is not limited to any particular programminglanguage or operating system.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly, it should be appreciated that in the above description ofexemplary embodiments of the invention, various features of theinvention are sometimes grouped together in a single embodiment, figure,or description thereof for the purpose of streamlining the disclosureand aiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the Detailed Description are hereby expressly incorporatedinto this Detailed Description, with each claim standing on its own as aseparate embodiment of this invention.

Similarly, it should be appreciated that in the above description ofexemplary embodiments of the invention, various features of theinvention are sometimes grouped together in a single embodiment, figure,or description thereof for the purpose of streamlining the disclosureand aiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the Detailed Description are hereby expressly incorporatedinto this Detailed Description, with each claim standing on its own as aseparate embodiment of this invention.

Furthermore, some of the embodiments are described herein as a method orcombination of elements of a method that can be implemented by aprocessor of a computer system or by other means of carrying out thefunction. Thus, a processor with the necessary instructions for carryingout such a method or element of a method forms one example of a meansfor carrying out the method or element of the method. Furthermore, anelement described herein of an apparatus embodiment is one example of ameans for carrying out the function performed by the element for thepurpose of carrying out the invention.

All publications, patents, and patent applications cited herein arehereby incorporated by reference.

In the claims below and the description herein, the term “comprising” or“comprised of” or “which comprises” is an “open” term that meansincluding at least the elements/features that follow, but not excludingothers. The term “including” or “which includes” or “that includes” asused herein is also an “open” term that also means including at leastthe elements/features that follow the term, but not excluding others.Thus, including is synonymous with and means comprising.

Thus, while there has been described what are believed to be thepreferred embodiments of the invention, those skilled in the art willrecognize that other and further modifications may be made theretowithout departing from the spirit of the invention, and it is intendedto claim all such changes and modifications as fall within the scope ofthe invention. For example, any formulas given above are merelyrepresentative of procedures that may be used. Functionality may beadded or deleted from the block diagrams and operations may beinterchanged among functional blocks. Steps may be added or deleted tomethods described within the scope of the present invention.Furthermore, the words comprising and comprise are meant in the sense of“including” and “include” so describe including at least the elements orsteps described, and provide for additional elements or steps.

1. A method at a first wireless station of transmitting to a secondwireless station, the first and second wireless stations forcommunicating packets of information using OFDM signals that include aplurality of frequency tones, the first station including a plurality ofantennas for receiving and transmitting coupled to a correspondingplurality of receive signal paths for receiving and to a correspondingplurality of transmit signal paths for transmitting, the methodcomprising: determining the channel response for each receive signalpath, the channel response determining using signals received at thefirst station corresponding to a part of a packet transmitted from thesecond station, the part of the packet having known values for a set oftones, the channel determining including performing a discrete Fouriertransform to determine received tones corresponding to the part of thepacket and generating channel estimates for the receive signal paths foreach tone whose value is known in the part of the packet; determining aset of transmit weights for each tone for each transmit signal path, thetransmit weight set determining using the channel estimates;tone-by-tone weighting a signal for transmission by the set ofdetermined transmit weights to produce weighted tone sets fortransmission via each transmit signal path; inverse discrete Fouriertransforming the weighted tone sets to produce antenna signals fortransmission via the transmit signal paths; and transmitting the set ofantenna signals for transmission via each transmit signal path via theantennas, wherein the first station is configured such that the channelresponse at the receiving second station includes an additivecontribution for transmissions via each transmitting antenna of thefirst station, such that the second station can receive the signal fortransmission without the second station requiring a plurality of receiveantennas and without any first-station-specific calibration required atthe second station.
 2. A method as recited in claim 1, wherein thenumber of antennas is two, such that there are two transmit and tworeceive signal paths.
 3. A method as recited in claim 1, wherein thepackets substantially conform to one of the OFDM variants of the IEEE802.11 standard or derivatives thereof, wherein each packet includes apreamble having at least one long symbol having known tones, and whereinthe packet part having the known tones includes at least one of the longsymbols.
 4. A method as recited in claim 3, wherein the first station isan access point.
 5. A method as recited in claim 3, wherein the firststation is a client station.
 6. A method as recited in claim 3, whereinthe determining of the channel responses of the receive signal pathincludes: for each received tone corresponding to a long symbol,determining a rough estimated channel response; and smoothing the roughestimated channel response for a tone with the rough estimated channelresponses of neighboring tones.
 7. A method as recited in claim 3,wherein the packet includes two long symbols and wherein determining ofthe channel responses of the receive signal path includes: averaging thechannel responses calculated from received tones corresponding to eachlong symbol.
 8. A method as recited in claim 3, wherein the transmitweights determining and the tone-by-tone weighting together comprise:for each tone, selecting one of the transmit signal paths fortransmitting the signal for transmitting, the selecting according to thedetermined channel response that has the largest magnitude, such thatfor each tone, the selecting is equivalent to weighting the signal fortransmitting via the selected transmit signal path by one, and weightingthe signal for transmitting via each other transmit signal path by zero.9. A method as recited in claim 1, wherein the transmit weight for eachtransmit signal path corresponding to each antenna has a phase anglewhich is the negative of the phase angle of the determined channelresponse for a corresponding receive signal path connected to the sameantenna.
 10. A method as recited in claim 9, wherein each transmitsignal path of the first station includes a transmit digital signal pathwhose output is coupled to a digital-to-analog converter whose output iscoupled to a transmit RF signal path coupled to the antennacorresponding to the transmit signal path, such that the magnitude andphase response of the transmit digital signal path is substantially thesame for each transmit signal path, wherein the first station isconfigured such that the magnitude and phase response of each RFtransmit signal path is substantially equal.
 11. A method as recited inclaim 10, wherein each receive signal path of the first station includesa receive RF signal path coupled to the antenna corresponding to thereceive signal path, the output of each receive RF signal path coupledto an analog-to-digital converter whose output is coupled to a receivedigital signal path, such that the magnitude and phase response of thereceive digital signal path is substantially the same for each receivesignal path, and wherein the first station is configured such that thephase response of each RF transmit signal path is substantially equal.12. A method at a first wireless station of transmitting to a secondwireless station, the first and second wireless stations forcommunicating packets of information using OFDM signals that include aplurality of frequency tones, the first station including a plurality ofantennas for receiving and transmitting coupled to a transmit selectorcoupled to a transmit signal path for transmitting, the methodcomprising: determining a measure of the EVM for signals received fromthe second station via each of the antennas of the first station;selecting one of the transmit antennas for transmitting to the secondstation based on the measure of the EVM signals received from the secondstation; and transmitting a signal for transmission to the secondstation via the selected antenna, such that the second station canreceive the signal for transmission without the second station requiringa plurality of receive antennas and without any first-station-specificcalibration required at the second station.
 13. A method as recited inclaim 12, wherein the number of antennas is two.
 14. A method as recitedin claim 12, wherein the packets substantially conform to one of theOFDM variants of the IEEE 802.11 standard or derivatives thereof,wherein each packet includes a preamble, and wherein the determining ofthe measures of the EVM via each antenna occurs during receiving at thefirst station of signals corresponding to the preamble of a packettransmitted from the second station.
 15. A method as recited in claim14, wherein the first station is an access point.
 16. A method asrecited in claim 14, wherein the first station is a client station. 17.A method as recited in claim 14, wherein the first station includes areceive antenna selector coupled to each of the antennas, and whereinthe determining of the measure of the EVM includes sequentiallyswitching between each of the antennas during receiving of the preamblepart to determine the measure of the EVM for the signals received fromthe second station via each of the antennas.
 18. A method as recited inclaim 17, wherein the sequentially switching between each of theantennas during receiving is prior to conducting automatic gain controlat the first station.
 19. A method as recited in claim 14, wherein thefirst station includes a receive signal path coupled to each antenna,and wherein the determining of the measure of the EVM includesdetermining the measure of the EVM for the signals received from thesecond station via each of the receive signal paths corresponding toeach of the antennas.
 20. A method at a first wireless station oftransmitting to a second wireless station, the first and second wirelessstations for communicating packets of information using OFDM signalsthat include a plurality of frequency tones, the first station includinga plurality of antennas for receiving and transmitting coupled to acorresponding plurality of receive signal paths for receiving and to acorresponding plurality of transmit signal paths for transmitting, themethod comprising: determining the channel response for each receivesignal path, the channel response determining using signals received atthe first station corresponding to a part of a packet transmitted fromthe second station, the part of the packet having known values for a setof tones, the channel determining including performing a discreteFourier transform to determine received tones corresponding to the partof the packet and generating channel estimates for the receive signalpaths for each tone whose value is known in the part of the packet; foreach tone, selecting one of the transmit signal paths for transmittingthe signal for transmitting, the selecting according to the determinedchannel response that has the largest magnitude, such that the selectingis equivalent to weighting the signal for transmitting via the selectedtransmit signal path by one, and weighting the signal for transmittingvia each other transmit signal path by zero forming a weighted tone setfor each transmit signal path; inverse discrete Fourier transforming theweighted tone sets to produce antenna signals for transmission via thetransmit signal paths; and transmitting the set of antenna signals fortransmission via each transmit signal path via the antennas, such thatthe second station can receive the signal for transmission without thesecond station requiring a plurality of receive antennas and without anyfirst-station-specific calibration required at the second station.
 21. Amethod as recited in claim 20, wherein the number of antennas is two,such that there are two transmit and two receive signal paths.
 22. Amethod as recited in claim 20, wherein the packets substantially conformto one of the OFDM variants of the IEEE 802.11 standard or derivativesthereof, wherein each packet includes a preamble having at least onelong symbol having known tones, and wherein the packet part having theknown tones includes at least one of the long symbols.
 23. A method asrecited in claim 22, wherein the first station is an access point.
 24. Amethod as recited in claim 22, wherein the first station is a clientstation.
 25. A method as recited in claim 22, wherein determining of thechannel responses of the receive signal path includes: for each receivedtone corresponding to a long symbol, determining a rough estimatedchannel response; and smoothing the rough estimated channel response fora tone with the rough estimated channel responses of neighboring tones.26. A method as recited in claim 22, wherein the packet includes twolong symbols and wherein determining of the channel responses of thereceive signal path includes: averaging the channel responses calculatedfrom received tones corresponding to each long symbol.
 27. An apparatusin a first wireless station for transmitting to a second wirelessstation, the first and second wireless stations for communicatingpackets of information using OFDM signals that include a plurality offrequency tones, the first station including a plurality of antennas forreceiving and transmitting coupled to a corresponding plurality ofreceive signal paths for receiving and to a corresponding plurality oftransmit signal paths for transmitting, the apparatus comprising: meansfor determining the channel response for each receive signal path, themeans for channel response determining accepting signals received at thefirst station corresponding to a part of a packet transmitted from thesecond station, the part of the packet having known values for a set oftones, the channel determining means including means for performing adiscrete Fourier transform to determine received tones corresponding tothe part of the packet and means for generating channel estimates forthe receive signal paths for each tone whose value is known in the partof the packet; means for determining a set of transmit weights for eachtone for each transmit signal path, the transmit weight set determiningusing the channel estimates; means for tone-by-tone weighting a signalfor transmission by the set of determined transmit weights to produceweighted tone sets for transmission via each transmit signal path; meansfor inverse discrete Fourier transforming the weighted tone sets toproduce antenna signals for transmission via the transmit signal paths;and means for transmitting the set of antenna signals for transmissionvia each transmit signal path via the antennas, wherein the firststation is configured such that the channel response at the receivingsecond station includes an additive contribution for transmissions viaeach transmitting antenna of the first station, such that the secondstation can receive the signal for transmission without the secondstation requiring a plurality of receive antennas and without anyfirst-station-specific calibration required at the second station. 28.An apparatus as recited in claim 27, wherein the number of antennas istwo, such that there are two transmit and two receive signal paths. 29.An apparatus as recited in claim 27, wherein the packets substantiallyconform to one of the OFDM variants of the IEEE 802.11 standard orderivatives thereof, wherein each packet includes a preamble having atleast one long symbol having known tones, and wherein the packet parthaving the known tones includes at least one of the long symbols.
 30. Anapparatus as recited in claim 29, wherein the first station is an accesspoint.
 31. An apparatus as recited in claim 29, wherein the firststation is a client station.
 32. An apparatus as recited in claim 29,wherein the means for determining the channel responses of the receivesignal path includes: means for determining a rough estimated channelresponse for each received tone corresponding to a long symbol; andmeans for smoothing the rough estimated channel response for a tone withthe rough estimated channel responses of neighboring tones.
 33. Anapparatus as recited in claim 29, wherein the packet includes two longsymbols and wherein the means for determining the channel responses ofthe receive signal path includes: means for averaging the channelresponses calculated from received tones corresponding to each longsymbol.
 34. An apparatus as recited in claim 29, wherein the means fordetermining transmit weights and the means for tone-by-tone weightingtogether comprise: means for selecting one of the transmit signal pathsfor transmitting the signal for transmitting, the means for selectingaccording to the determined channel response that has the largestmagnitude, such that for each tone, the means for selecting weights thesignal for transmitting via the selected transmit signal path by one,and weights the signal for transmitting via each other transmit signalpath by zero.
 35. An apparatus as recited in claim 27, wherein thetransmit weight for each transmit signal path corresponding to eachantenna has a phase angle which is the negative of the phase angle ofthe determined channel response for a corresponding receive signal pathconnected to the same antenna.
 36. An apparatus as recited in claim 35,wherein each transmit signal path of the first station includes atransmit digital signal path whose output is coupled to means fordigital-to-analog converting whose output is coupled to a transmit RFsignal path coupled to the antenna corresponding to the transmit signalpath, such that the magnitude and phase response of the transmit digitalsignal path is substantially the same for each transmit signal path,wherein the first station is configured such that the magnitude andphase response of each RF transmit signal path is substantially equal.37. An apparatus as recited in claim 36, wherein each receive signalpath of the first station includes a receive RF signal path coupled tothe antenna corresponding to the receive signal path, the output of eachreceive RF signal path coupled to means for analog-to-digital convertingwhose output is coupled to a receive digital signal path, such that themagnitude and phase response of the receive digital signal path issubstantially the same for each receive signal path, and wherein thefirst station is configured such that the phase response of each RFtransmit signal path is substantially equal.
 38. An apparatus in a firstwireless station for transmitting to a second wireless station, thefirst and second wireless stations for communicating packets ofinformation using OFDM signals that include a plurality of frequencytones, the first station including a plurality of antennas for receivingand transmitting coupled to a transmit selector coupled to a transmitsignal path for transmitting, the apparatus comprising: means fordetermining a measure of the EVM for signals received from the secondstation via each of the antennas of the first station; means forselecting one of the transmit antennas for transmitting to the secondstation, the selecting means accepting the measures of the EVM signalsreceived from the second station for selecting the one transmit antenna;and means for transmitting a signal for transmission to the secondstation via the antenna selected by the means for selecting, such thatthe second station can receive the signal for transmission without thesecond station requiring a plurality of receive antennas and without anyfirst-station-specific calibration required at the second station. 39.An apparatus as recited in claim 38, wherein the number of antennas istwo.
 40. An apparatus as recited in claim 38, wherein the packetssubstantially conform to one of the OFDM variants of the IEEE 802.11standard or derivatives thereof, wherein each packet includes apreamble, and wherein the determining of the measures of the EVM viaeach antenna occurs during receiving at the first station of signalscorresponding to the preamble of a packet transmitted from the secondstation.
 41. An apparatus as recited in claim 40, wherein the firststation is an access point.
 42. An apparatus as recited in claim 40,wherein the first station is a client station.
 43. An apparatus asrecited in claim 40, wherein the first station includes means forselecting a receive antenna, the receive antenna selecting means coupledto each of the antennas, and wherein the means for determining themeasure of the EVM sequentially switches between each of the antennasduring receiving of the preamble part to determine the measure of theEVM for the signals received from the second station via each of theantennas.
 44. An apparatus as recited in claim 43, wherein thesequential switching between each of the antennas during receiving isprior to conducting automatic gain control at the first station.
 45. Anapparatus as recited in claim 40, wherein the first station includes areceive signal path coupled to each antenna, and wherein the means fordetermining the measure of the EVM includes means for determining themeasure of the EVM for the signals received from the second station viaeach of the receive signal paths corresponding to each of the antennas.46. An apparatus in a first wireless station for transmitting to asecond wireless station, the first and second wireless stations forcommunicating packets of information using OFDM signals that include aplurality of frequency tones, the first station including a plurality ofantennas for receiving and transmitting coupled to a correspondingplurality of receive signal paths for receiving and to a correspondingplurality of transmit signal paths for transmitting, the apparatuscomprising: means for determining the channel response for each receivesignal path, the channel response determining means using signalsreceived at the first station corresponding to a part of a packettransmitted from the second station, the part of the packet having knownvalues for a set of tones, the channel determining means including meansfor performing a discrete Fourier transform to determine received tonescorresponding to the part of the packet and means for generating channelestimates for the receive signal paths for each tone whose value isknown in the part of the packet; means for selecting one of the transmitsignal paths for each tone for transmitting the signal for transmitting,the means for selecting according to the determined channel responsethat has the largest magnitude, such that the means for selecting isequivalent to means for weighting the signal for transmitting via theselected transmit signal path by one, and weighting the signal fortransmitting via each other transmit signal path by zero forming aweighted tone set for each transmit signal path; means for inversediscrete Fourier transforming the weighted tone sets to produce antennasignals for transmission via the transmit signal paths; and means fortransmitting the set of antenna signals for transmission via eachtransmit signal path via the antennas, such that the second station canreceive the signal for transmission without the second station requiringa plurality of receive antennas and without any first-station-specificcalibration required at the second station.
 47. An apparatus as recitedin claim 46, wherein the number of antennas is two, such that there aretwo transmit and two receive signal paths.
 48. An apparatus as recitedin claim 46, wherein the packets substantially conform to one of theOFDM variants of the IEEE 802.11 standard or derivatives thereof,wherein each packet includes a preamble having at least one long symbolhaving known tones, and wherein the packet part having the known tonesincludes at least one of the long symbols.
 49. An apparatus as recitedin claim 48, wherein the first station is an access point.
 50. Anapparatus as recited in claim 48, wherein the first station is a clientstation.
 51. An apparatus as recited in claim 48, wherein the means fordetermining of channel responses of the receive signal path includes:means for determining a rough estimated channel response for eachreceived tone corresponding to a long symbol; and means for smoothingthe rough estimated channel response for a tone with the rough estimatedchannel responses of neighboring tones.
 52. An apparatus as recited inclaim 48, wherein the packet includes two long symbols and wherein themeans for determining the channel responses of the receive signal pathincludes: means for averaging the channel responses calculated fromreceived tones corresponding to each long symbol.
 53. An apparatus in afirst wireless station for transmitting to a second wireless station,the first and second wireless stations for communicating packets ofinformation using OFDM signals that include a plurality of frequencytones, the first station including a plurality of antennas for receivingand transmitting coupled to a corresponding plurality of receive signalpaths for receiving and to a corresponding plurality of transmit signalpaths for transmitting, the apparatus comprising: in each receive signalpath: a radio receiver coupled to the antenna of the receive signalpath; an analog-to-digital converter coupled to the radio receiver toproduce a received signal for the antenna of the receive signal path; adiscrete Fourier transformer coupled to the analog-to-digital converterto output tones corresponding to the signal received at the antenna ofthe receive signal path; and a channel estimator coupled to the discreteFourier transformer, the channel estimator accepting received tonescorresponding to a part of a packet transmitted from the second stationand received by the radio receiver, the part of the packet having knownvalues for a set of tones, the channel estimator configured to determinechannel estimates for the receive signal path for each tone whose valueis known in the part of the packet; the apparatus further comprising: aweight calculator coupled to each receive signal path's channelestimator and configured to determine a set of transmit weights for eachtone for each transmit signal path, the transmit weight set determiningusing the channel estimates; a transmit beamformer coupled to the weightcalculator configured to tone-by-tone weight a signal for transmissionby the set of determined transmit weights to produce weighted tone setsfor transmission via each transmit signal path; and for each transmitsignal path: an inverse discrete Fourier transformer coupled to thetransmit beamformer and configured to produce from the transmit signalpath's weighted tone set an antenna signal for transmission via thetransmit signal path; a digital-to-analog converter coupled to theinverse discrete Fourier transformer to convert the antenna signal fortransmission to an analog signal for transmission; and a radiotransmitter coupled to the digital-to-analog converter configured toaccept the analog signal for transmission and transmit the analog signalvia the transmit signal paths antenna, wherein the first station isconfigured such that the channel response at the receiving secondstation includes an additive contribution for transmissions via eachtransmitting antenna of the first station, such that the second stationcan receive the signal for transmission without the second stationrequiring a plurality of receive antennas and without anyfirst-station-specific calibration required at the second station. 54.An apparatus as recited in claim 53, wherein the number of antennas istwo, such that there are two transmit and two receive signal paths. 55.An apparatus as recited in claim 53, wherein the packets substantiallyconform to one of the OFDM variants of the IEEE 802.11 standard orderivatives thereof, wherein each packet includes a preamble having atleast one long symbol having known tones, and wherein the packet parthaving the known tones includes at least one of the long symbols.
 56. Anapparatus as recited in claim 55, wherein the first station is an accesspoint.
 57. An apparatus as recited in claim 55, wherein the firststation is a client station.
 58. An apparatus as recited in claim 55,wherein each channel estimator is configured to determine the channelresponse of the receive signal path including: determining a roughestimated channel response for each received tone corresponding to along symbol; and smoothing the rough estimated channel response for atone with the rough estimated channel responses of neighboring tones.59. An apparatus as recited in claim 55, wherein the packet includes twolong symbols and wherein each channel estimator is configured todetermine the channel response of the receive signal path including:averaging the channel responses calculated from received tonescorresponding to each long symbol.
 60. An apparatus as recited in claim55, wherein the weight calculator and transmit beamformer are configuredto, for each tone, select one of the transmit signal paths fortransmitting the signal for transmitting according to the determinedchannel response that has the largest magnitude, such that for eachtone, the selecting weights the signal for transmitting via the selectedtransmit signal path by one, and weights the signal for transmitting viaeach other transmit signal path by zero.
 61. An apparatus as recited inclaim 53, wherein the weight calculator is configured to determine atransmit weight for each transmit signal path that has a phase anglewhich is the negative of the phase angle of the determined channelresponse for the corresponding receive signal path connected to the sameantenna.
 62. An apparatus as recited in claim 61, wherein the radiotransmitters are configured to have substantially the same magnitude andphase response.
 63. An apparatus as recited in claim 62, wherein theradio receivers are configured to have substantially the same phaseresponse.
 64. An apparatus in a first wireless station for transmittingto a second wireless station, the first and second wireless stations forcommunicating packets of information using OFDM signals that include aplurality of frequency tones, the first station including a plurality ofantennas for receiving and transmitting coupled to an antenna selector,the antenna selector coupled to a transmit signal path for transmittingand to a receive signal path for receiving, the apparatus comprising: aradio receiver coupled to the selector to receive a signal; ananalog-to-digital converter coupled to the radio receiver output toproduce a digital received signal; an EVM calculator coupled to theanalog-to-digital converter and coupled to the antenna selector, the EVMcalculator configured to determine a measure of the EVM of signalsreceived from the second station via each of the antennas of the firststation; and a radio transmitter accepting a signal for transmitting andcoupled to the selector, the radio transmitter configured to transmitvia the antenna selected by the selector, wherein the EVM calculator isconfigured to generate a transmit selector signal for selecting onetransmit antenna for transmitting to the second station, such that thesecond station can receive the signal for transmission without thesecond station requiring a plurality of receive antennas and without anyfirst-station-specific calibration required at the second station. 65.An apparatus as recited in claim 64, wherein the number of antennas istwo.
 66. An apparatus as recited in claim 64, wherein the packetssubstantially conform to one of the OFDM variants of the IEEE 802.11standard or derivatives thereof, wherein each packet includes apreamble, and wherein the determining the measures of the EVM via eachantenna occurs during receiving at the first station of signalscorresponding to the preamble of a packet transmitted from the secondstation.
 67. An apparatus as recited in claim 66, wherein the firststation is an access point.
 68. An apparatus as recited in claim 66,wherein the first station is a client station.
 69. An apparatus asrecited in claim 66, wherein the EVM calculator and selector further areconfigured to sequentially switch between each of the antennas duringreceiving of the preamble part to determine the measure of the EVM forthe signals received from the second station via each of the antennas.70. An apparatus as recited in claim 69, wherein the sequentialswitching between each of the antennas during receiving is prior toconducting automatic gain control at the first station.
 71. An apparatusin a first wireless station for transmitting to a second wirelessstation, the first and second wireless stations for communicatingpackets of information using OFDM signals that include a plurality offrequency tones, the first station including a plurality of antennas forreceiving and transmitting coupled to a corresponding plurality ofreceive signal paths for receiving and to a corresponding plurality oftransmit signal paths for transmitting, the apparatus comprising: ineach receive signal path: a radio receiver coupled to the antenna of thereceive signal path; an analog-to-digital converter coupled to the radioreceiver to produce a received signal for the antenna of the receivesignal path; a discrete Fourier transformer coupled to theanalog-to-digital converter to output tones corresponding to the signalreceived at the antenna of the receive signal path; and a channelestimator coupled to the discrete Fourier transformer, the channelestimator accepting received tones corresponding to a part of a packettransmitted from the second station and received by the radio receiver,the part of the packet having known values for a set of tones, thechannel estimator configured to determine channel estimates for thereceive signal path for each tone whose value is known in the part ofthe packet; the apparatus further comprising: a transmit weightcalculator/beamformer coupled to each receive signal path's channelestimator and configured to select one of the transmit signal paths foreach tone for transmitting, the selecting according to the determinedchannel response that has the largest magnitude, such that the weightcalculator/beamformer weights the signal for transmitting via theselected transmit signal path by one, and weighting the signal fortransmitting via each other transmit signal path by zero forming aweighted tone set for each transmit signal path; and for each transmitsignal path: an inverse discrete Fourier transformer coupled to thetransmit weight calculator/beamformer and configured to produce from thetransmit signal path's weighted tone set an antenna signal fortransmission via the transmit signal path; and a digital-to-analogconverter coupled to the inverse discrete Fourier transformer to convertthe antenna signal for transmission to an analog signal fortransmission; and a radio transmitter coupled to the digital-to-analogconverter configured to accept the analog signal for transmission andtransmit the analog signal via the transmit signal paths antenna, suchthat the second station can receive the signal for transmission withoutthe second station requiring a plurality of receive antennas and withoutany first-station-specific calibration required at the second station.72. An apparatus as recited in claim 71, wherein the number of antennasis two, such that there are two transmit and two receive signal paths.73. An apparatus as recited in claim 71, wherein the packetssubstantially conform to one of the OFDM variants of the IEEE 802.11standard or derivatives thereof, wherein each packet includes a preamblehaving at least one long symbol having known tones, and wherein thepacket part having the known tones includes at least one of the longsymbols.
 74. An apparatus as recited in claim 73, wherein the firststation is an access point.
 75. An apparatus as recited in claim 73,wherein the first station is a client station.
 76. An apparatus asrecited in claim 73, wherein each channel estimator is configured todetermine the channel response of the receive signal path including:determining a rough estimated channel response for each received tonecorresponding to a long symbol; and smoothing the rough estimatedchannel response for a tone with the rough estimated channel responsesof neighboring tones.
 77. An apparatus as recited in claim 73, whereinthe packet includes two long symbols and wherein each channel estimatoris configured to determine the channel response of the receive signalpath including: averaging the channel responses calculated from receivedtones corresponding to each long symbol.
 78. A carrier medium carryingcomputer readable code segments to instruct one or more processors of aprocessing system to execute a method at a first wireless station oftransmitting to a second wireless station, the first and second wirelessstations for communicating packets of information using OFDM signalsthat include a plurality of frequency tones, the first station includinga plurality of antennas for receiving and transmitting coupled to acorresponding plurality of receive signal paths for receiving and to acorresponding plurality of transmit signal paths for transmitting, themethod comprising: determining the channel response for each receivesignal path, the channel response determining using signals received atthe first station corresponding to a part of a packet transmitted fromthe second station, the part of the packet having known values for a setof tones, the channel determining including performing a discreteFourier transform to determine received tones corresponding to the partof the packet and generating channel estimates for the receive signalpaths for each tone whose value is known in the part of the packet;determining a set of transmit weights for each tone for each transmitsignal path, the transmit weight set determining using the channelestimates; tone-by-tone weighting a signal for transmission by the setof determined transmit weights to produce weighted tone sets fortransmission via each transmit signal path; inverse discrete Fouriertransforming the weighted tone sets to produce antenna signals fortransmission via the transmit signal paths; and transmitting the set ofantenna signals for transmission via each transmit signal path via theantennas, wherein the first station is configured such that the channelresponse at the receiving second station includes an additivecontribution for transmissions via each transmitting antenna of thefirst station, such that the second station can receive the signal fortransmission without the second station requiring a plurality of receiveantennas and without any first-station-specific calibration required atthe second station.
 79. A carrier medium as recited in claim 78, whereinthe number of antennas is two, such that there are two transmit and tworeceive signal paths.
 80. A carrier medium as recited in claim 78,wherein the packets substantially conform to one of the OFDM variants ofthe IEEE 802.11 standard or derivatives thereof, wherein each packetincludes a preamble having at least one long symbol having known tones,and wherein the packet part having the known tones includes at least oneof the long symbols.
 81. A carrier medium as recited in claim 80,wherein the transmit weights determining and the tone-by-tone weightingtogether comprise: for each tone, selecting one of the transmit signalpaths for transmitting the signal for transmitting, the selectingaccording to the determined channel response that has the largestmagnitude, such that for each tone, the selecting is equivalent toweighting the signal for transmitting via the selected transmit signalpath by one, and weighting the signal for transmitting via each othertransmit signal path by zero.
 82. A carrier medium as recited in claim78, wherein the transmit weight for each transmit signal pathcorresponding to each antenna has a phase angle which is the negative ofthe phase angle of the determined channel response for a correspondingreceive signal path connected to the same antenna.